(1) Field of the Invention
The present invention relates to the fabrication of integrated circuits, and more particularly to a method for forming a novel graded silicon nitride (Si3N4)/silicon oxide (SiO2) (also referred to as SNO) hard mask on a substrate. This novel SNO layer improves the profile (optical fidelity) of the photoresist etch mask image used to pattern the SNO layer, and thereby improves the critical dimensions (CD) for deep submicrometer semiconductor circuits.
(2) Description of the Prior Art
As the minimum feature sizes continue to decrease to deep submicrometer (um) dimensions (0.10-0.20 um) for making Ultra Large Scale Integration (ULSI) circuits, it is necessary to further improve the fidelity of the photoresist mask images used to etch the various material layers formed on semiconductor substrates. One of these layers is the silicon nitride (Si3N4) used as an oxidation barrier and/or hard mask on a single-crystal silicon substrate. In the conventional process a relatively thin pad oxide (e.g.,SiO2) is formed on the surface of the silicon substrate, and a Si3N4 layer is deposited. The Si3N4 is patterned using a photoresist mask and plasma etching to leave portions of the Si3N4 over the device areas while removing portions on the silicon substrate surface around the device areas. A field oxide is formed around the device areas to electrically isolate the individual device areas from each other. The interposing pad oxide serves to reduce the high stress from the Si3N4 which would otherwise cause crystalline defects in the single-crystal silicon. In early versions of the process, the field oxide was formed by thermally oxidizing the silicon in the field oxide areas, while the patterned Si3N4 protected the device areas from oxidation. This conventional approach is commonly referred to in the industry as the LOCal Oxidation of Silicon (LOCOS) process. Unfortunately, the LOCOS process is an essentially isotropic oxidation and results in lateral oxidation under the Si3N4 oxidation barrier mask, commonly referred to as xe2x80x9cthe bird""s beakxe2x80x9d because of it shape. This lateral oxidation is sensitive to the thickness of the pad oxide and limits the minimum feature size device areas. If the pad oxide is too thin, the stress from Si3N4 hard mask is unacceptably high causing silicon defects, and if the pad oxide is too thick the bird""s beak extends too far under the oxidation barrier mask and limits the pattern density. Kobayashi et al. in U.S. Pat. No. 5,616,401 teaches several methods of reducing the bird""s beak using a graded silicon oxynitride/silicon nitride isolation oxidation mask. In U.S. Pat. No. 5,510,290 to Kwon another approach is described. Kwon etches openings in a Si3N4 hard mask to a thin thermal silicon oxide over the field oxide areas and thermally converts the oxide to silicon oxynitride (SiON). Sidewall spacers are formed in the openings to reduce the width, and the SiON is etched to the silicon substrate, and a LOCOS is performed to crate narrow field oxides. Hyung et al. in U.S. Pat. No. 5,523,255 uses a polysilicon buffer layer between the pad oxide and the silicon nitride hard mask to reduce the length of the bird""s beak. In U.S. Pat. No. 5,629,043, Inaba et al. teach a method for making a better thin Si3N4 interelectrode dielectric layer for stacked DRAM capacitors.
An alternative approach of making a field oxide isolation which eliminates the bird""s beak and increases the density of device areas is to form a field oxide isolation, commonly referred to as a shallow trench isolation (STI). This approach eliminates the bird""s beak and is more desirable for deep submicrometer processing. In this method a Si3N4 and the photoresist mask protect the device areas while the exposed silicon is anisotropically etched in the field oxide areas to form trenches having essentially vertical sidewalls. After removing the photoresist it is common practice to grow a sacrificial oxide and wet etch the oxide to remove any silicon damaged due to the plasma etching, and then a new thin oxide is grown to provide a good interface at the silicon surface. The trench is then filled with a chemical vapor deposited (CVD) silicon oxide and etched or polished back to the Si3N4 hard mask. This STI structure circumvents the bird""s beak problem and provides a planar substrate surface that is ideal for subsequent processing steps.
However, to achieve deep submicrometer feature sizes it is necessary to make reliable and reproducible photoresist mask images on the Si3N4 hard mask. One problem associated with making reliable well shaped photoresist images is best understood by referring to the prior art of FIG. 1. In FIG. 1 a silicon substrate 10 typically has a pad oxide 12 formed on the surface to reduce stress, and then a Si3N4 layer 14 is deposited. A blanket photoresist layer 16 is spin coated and is optically exposed and developed to leave portions of the photoresist over device areas 2 on the substrate 10, while exposing the Si3N4 14 in the areas 4. This photoresist mask and plasma etching are then used to etch the Si3N4 and trenches in the substrate for the STI field oxide. To etch deep submicrometer structures the patterned photoresist layer 16 should have an essentially vertical profile down to the Si3N4 hard mask layer 14. However, because of interactions between the patterned photoresist layer 16 and the nitride layer when exposed with the short wavelength deep ultraviolet (DUV) light, unwanted undercutting or lateral recessing 6 occurs in the photoresist at the Si3N4 surface during photoresist developing. These recesses 6 make it difficult to etch reliable submicrometer images in the Si3N4 layer 14. To avoid this problem it is a common practice in the semiconductor industry to plasma treat the surface with oxygen (O2) prior to patterning the photoresist.
One method of making shallow trench isolation (STI) is described in U.S. Pat. No. 5,863,827 to Joyner. The invention utilizes a method for making rounded trench corners to minimize the electric field effects when the circuit is powered on. However, Joyner does not address the need for forming photoresist patterns which are free of recesses 6 as described above with reference to FIG. 1.
However, there is still a need in the semiconductor industry to improve the Si3N4 hard mask etch process while providing a more cost-effective manufacturing process.
A principal object of this invention is to make a novel graded silicon nitride-silicon oxide (SNO) hard mask. This novel hard mask layer reduces the reflective index of the hard mask, and allows photoresist images to be formed on the hard mask layer with an improved shape. The photoresist image is formed having essentially vertical sidewalls and without recesses formed in the photoresist mask at the photoresist hard-mask layer interface, that would otherwise occur in the conventional process, as depicted in the prior art FIG. 1.
The method for making this improved graded composite silicon nitride-silicon oxide (SNO) hard mask begins by providing a semiconductor substrate. The most commonly used substrate is a single-crystal silicon (Si) substrate. A stress release layer is formed on the principal surface of the silicon substrate. One method of forming a stress release layer is to thermally oxidize the silicon surface to form a thin SiO2 layer. The graded composite silicon nitride-silicon oxide (SNO) hard mask layer is then formed by depositing a silicon nitride (Si3N4) film using a chemical vapor deposition tool and reactant gases of silane (SiH4) and ammonia (NH3). During the Si3N4 film deposition the flow rate of the NH3 reactant gas is continuously reduced while a third reactant gas of nitrous oxide (N2O) is introduced to form a silicon oxynitride (SiON) film. This results in a graded composite SNO layer which is nitrogen-rich (Si3N4) at the substrate surface and is continuously converted to a silicon oxynitride (SiON) that is oxygen-rich (essentially SiO2) at the top surface of the SNO layer.
The SiON film on the top surface of the SNO layer will also reduce the reflective index. The introduction of N2O also reduces the hydrogen content which would otherwise react with the deep UV photoresist and cause notches in the bottom of the photoresist. This novel graded composite silicon nitride-silicon oxide (SNO) layer eliminates the need to plasma treat in oxygen the Si3N4 hard mask layer used in the conventional process. A photoresist layer is deposited on the SNO layer by spin coating. Conventional processing is then used to optically expose the photoresist through a photomask (reticule), and the photoresist is developed to from the photoresist etch mask for etching the SNO layer. Since the adsorption constant of the SNO is lower and the hydrogen content on the top surface of the SNO are lower, the photoresist etch mask has vertical sidewalls. The need to use an additional plasma treatment on a conventional Si3N4 hard-mask layer to reduce the reflective index and to reduce the hydrogen content of the Si3N4 is eliminated. The photoresist etch masks can now be used for etching deep submicrometer patterns in the SNO layer.